TROUBLESHOOTING AND REPAIR
RF LEVEL/AM
Unleveled Condition 6D-8.
If the problem is in a specific frequency band (or bands) and other bands work
properly, check band control signals and band switches. See Table 6D-1 for band
control signal state definition and Table 6D-2 to determine pin diode states for various
frequency bands. A specific band (or bands) problem will most likely involve a divider,
a switch, a filter, or a control signal. If all of the frequency bands are affected, the
leveling loop or associated controls and inputs are probably at fault. First, check the
signal at the output connector (J16) of the Premodulator PCA. As the instrument
frequency is incremented from 15 to 1056 MHz, a corresponding signal should be seen
at J16. The levels typically range from +6 dBm at 15 MHz to +4 dBm at 1056 MHz with
a low of about 2.5 dBm at 256 Mhz. High harmonic levels will be seen in the frequencies
below 64 MHz. Lack of a proper signal here might involve U4, U5, or Q6 on the
Premodulator PCA. The input signal to the Premodulator PCA (at J5) from the Sum
Loop VCO (A9) should also be checked. The level here should be approximately +7
dBm.
If the Premodulator PCA output appears to be correct, the problem is on the
Modulation Control PCA, the Output PCA (between the input, W1, and the detector
diode, CR20), or possibly on the Controller PCA.
With the instrument programmed for SPCL 01, frequency set to 88 MHz, and level set
to 13 dBm, the voltage at TP7 (leveling loop control voltage) should be approximately
+ 1.7V DC. With the RF output programmed off, the voltage at TP7 should be 0V. If
these voltages are not correct, look at the Modulation Control PCA circuitry
associated with U21, U6, U8, U9, U11, U12, U4, or check inputs from the controller.
Table 6D-2. Frequency Band Logic States
6D-8
FREQUENCY BAND
.01 to 15-
15 to 22-
22 to 32-
32 to 47-
47 to 64-
64 to
128-
128 to
180-
180 to
256-
256 to
350-
350 to
512-
512 to
730-
730 to
1056
.01 to
625-
625 to
1056
CIRCUIT BOARD
Output, A8
Output, A8
Output, A8
Output, A8
Output, A8
Premodulator, A10
Premodulator, A10
Premodulator, A10
Premodulator, A10
Premodulator, A10
Premodulator, A10
Premodulator, A10
Output, A8
Output, A8
PIN DIODES TURNED "ON"
CR18, CR24
CR6, CR10, CR4, CR16
CR7, CR11, CR4, CR16
CR8, CR12, CR4, CR16
CR9, CR13, CR4, CR16
CR28, CR22, CR31
CR26, CR27, CR71, CR22, CR31
CR24, CR25, CR71, CR22, CR31
CR9, CR10, CR14, CR15, CR16
CR9, CR10, CR17, CR18, CR19
CR11, CR12, CR13, CR17, CR18, CR19
CR11, CR12, CR13, CR14, CR15, CR16
CR15
CR14
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